This invention relates to the field of alternator regulators and, more particularly, to a circuit for protecting the regulator in a vehicular alternator system.
In certain automobile ignition systems, the field obtains excitation current directly from the battery, which gives extremely good cut-in characteristics, but with a disadvantage--there is no element in the circuit which provides protection for the battery and the regulator. Therefore, if the ignition is switched on but the engine is not started, a heavy load will be put on the battery, and the regulator is quite likely to overheat since little or no cooling is provided under these "non-rotation" circumstances. It is normal for a switched excitation type of automotive voltage regulator to go to a 100% duty cycle operation when the input voltage drops slightly below the regulated voltage. However, since the field would have full excitation current of several amperes, it is possible to dissipate as much as 9 W in the regulator module and have a temperature rise of over 90.degree.0 C. Since the ambient temperature in the automotive environment may be as much as 120.degree. C., this means that the junction temperature of the Darlington drive transistor could be as much as 210.degree. C.
It is desirable to achieve protection against such an eventuality by adding a minimum of extra elements and it is possible to accomplish this by incorporating the present invention into a system which provides an indication of a number of possible alternator system faults. Such a system is disclosed in a co-pending application (Ser. No. 687,603), assigned to the assignee of the present invention, and it has, not only reliability, but essentially error-free operation. That is, temporary conditions which are not due to a fault requiring service are ignored, and the transient pulses which are normal to automobile ignition systems cannot cause false indications. The three main fault conditions which are detected and indicated are "non-rotation of the alternator", "overvoltage" and "undervoltage". For any of these three types of fault, a lamp will be lit with a single accompanying legend such as "CHECK ENGINE". When detecting an overvoltage condition, it is also important to distinguish between a transient overvoltage condition due, for example, to an abrupt drop in the load with a fully charged battery, and a true overvoltage condition due to an actual fault in the system such as a short-to-ground in the field coil.
When detecting an "undervoltage" condition, it is necessary to distinguish between a temporary low voltage, such as would occur when the load on the system exceeds the generating capacity and pulls the battery voltage down, and an actual fault such as a broken wire between the alternator and the battery.